8.1.2.1 Model Intercomparisons and Ensembles

The global model intercomparison activities that began in the late 1980s (e.g., Cess et al., 1989), and continued with the Atmospheric Model Intercomparison Project (AMIP), have now proliferated to include several dozen model intercomparison projects covering virtually all climate model components and various coupled model configurations (see http://www.clivar.org/science/mips.php for a summary). By far the most ambitious organised effort to collect and analyse Atmosphere-Ocean General Circulation Model (AOGCM) output from standardised experiments was undertaken in the last few years (see http://www-pcmdi.llnl.gov/ipcc/about_ipcc.php). It differed from previous model intercomparisons in that a more complete set of experiments was performed, including unforced control simulations, simulations attempting to reproduce observed climate change over the instrumental period and simulations of future climate change. It also differed in that, for each experiment, multiple simulations were performed by some individual models to make it easier to separate climate change signals from internal variability within the climate system. Perhaps the most important change from earlier efforts was the collection of a more comprehensive set of model output, hosted centrally at the Program for Climate Model Diagnosis and Intercomparison (PCMDI). This archive, referred to here as ‘The Multi-Model Data set (MMD) at PCMDI’, has allowed hundreds of researchers from outside the modelling groups to scrutinise the models from a variety of perspectives.

The enhancement in diagnostic analysis of climate model results represents an important step forward since the Third Assessment Report (TAR). Overall, the vigorous, ongoing intercomparison activities have increased communication among modelling groups, allowed rapid identification and correction of modelling errors and encouraged the creation of standardised benchmark calculations, as well as a more complete and systematic record of modelling progress.

Ensembles of models represent a new resource for studying the range of plausible climate responses to a given forcing. Such ensembles can be generated either by collecting results from a range of models from different modelling centres (‘multi-model ensembles’ as described above), or by generating multiple model versions within a particular model structure, by varying internal model parameters within plausible ranges (‘perturbed physics ensembles’). The approaches are discussed in more detail in Section 10.5.